Enforce coordinate eltype

[visr]

Do we currently expect anything from the coordinate eltype? Was SF always Float64? What do we do, <:Real, <:AbstractFloat, or leave it open?

[evetion]

I think it should be <:Real.

I will put this into the documentation, so any enforcement is not a breaking change later on. I'm also not sure how enforcement would look like or we want to specify an eltype in the interface itself.

[evetion]

This is already in the documentation itself:
GeoInterface.getcoord(geomtype(geom), geom::customgeom, i)::Real

I clarified it in the docstring as well:

    getcoord(geom, i) -> Number
Return the `i`th coordinate for a given `geom`. A coordinate isa *`Real`*.
Note that this is only valid for individual [`AbstractPointTrait`](@ref)s.

[visr]

Yeah indeed I think just documenting it might be enough, rather than "enforcing". Of course some operations may only make sense on AbstractFloat coordinates, but that is up to the implementer.

[evetion]

@rafaqz brought up the point to change Real into Number so things like Unitful.jl could work. This wouldn't be a breaking change.

[asinghvi17]

#128